What is Propagation Velocity in a Cable?

Cables 

What is Velocity Factor (Vf) or velocity of propagation (Vp) in a cable?

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Editorial Team - everything RF

Jul 22, 2019

The Velocity of propagation (Vp) of a cable is the speed at which an electrical signal can propagate through the cable in comparison to the speed of light. For example, in a vacuum, the velocity of propagation would be 100% or 1 (depending on how it is represented). So, if the Propagation Velocity is 70%, it means that the signal in a cable will propagate at 0.7 times the speed of a signal if the dielectric of the cable was vacuum.

The Velocity factor or Propagation Velocity of a cable can be calculated by the formula below (where ϵ is the dielectric constant of the dielectric material used in the cable).

The velocity of propagation or velocity factor is a coaxial cable is determined by the dielectric used. There are a number of materials that are used as the dielectric in a cable. Each of these dielectrics has a different dielectric constant and thus a different velocity factor. Here is a table of the most common dielectrics used in coax cables with their dielectric constant and velocity factor.

Dielectric Material
Dielectric Constant
Velocity Factor
Velocity of Propagation
Polyethylene (PE)
2.30.65965.9%
Foam Polyethylene
1.3 - 1.60.79 - 0.8879% to 88%
Air Spaced Polyethylene

0.84 - 0.8884% to 88%
Solid PTFE
2.070.69569.5%
Air Spaced PTFE    

0.85-0.9085% to 90%